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General Chemistry Principles, Patterns, and Applications, 2011

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Gray circles indicate the sizes of the ions shown; colored circles indicate the sizes of the neutral atoms, previously<br />

shown in Figure 7.7 "Calculated Atomic Radii (in Picometers) of the ".<br />

Source: Ionic radius data from R. D. Shannon, “Revised effective ionic radii <strong>and</strong> systematic studies<br />

of interatomic distances in halides <strong>and</strong> chalcogenides,” Acta Crystallographica 32, no. 5 (1976): 751–<br />

767.<br />

Note the Pattern<br />

Cations are always smaller than the neutral atom, <strong>and</strong> anions are always larger.<br />

Because most elements form either a cation or an anion but not both, there are few opportunities to compare the sizes<br />

of a cation <strong>and</strong> an anion derived from the same neutral atom. A few compounds of sodium, however, contain the<br />

Na − ion, allowing comparison of its size with that of the far more familiar Na + ion, which is found<br />

inmany compounds. The radius of sodium in each of its three known oxidation states is given in Table 7.2<br />

"Experimentally Measured Values for the Radius of Sodium in Its Three Known Oxidation States". All three species<br />

have a nuclear charge of +11, but they contain 10 (Na + ), 11 (Na 0 ), <strong>and</strong> 12 (Na − ) electrons. The Na + ion is significantly<br />

smaller than the neutral Na atom because the 3s 1 electron has been removed to give a closed shell with n = 2. The<br />

Na − ion is larger than the parent Na atom because the additional electron produces a 3s 2 valence electron<br />

configuration, while the nuclear charge remains the same.<br />

Table 7.2 Experimentally Measured Values for the Radius of Sodium in Its Three Known Oxidation States<br />

Na + Na 0 Na −<br />

Electron Configuration 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

608

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